1. Field of the Invention
The present invention is in the fields of molecular biology, cancer biology and medical therapeutics. The invention is generally directed to the identification of genes that inhibit the growth of, and induce apoptosis in, cancer cells, and to polypeptides encoded by such genes. In particular, the invention concerns the nucleotide sequence of one such tumor-inhibiting gene, tag7, the amino acid sequence of a polypeptide encoded by tag7, antibodies that specifically bind to the tag7 gene product, and methods of inhibiting cancer cell growth and of cancer therapy using the tag7 gene and gene product.
2. Related Art
Tumor Metastasis
The metastasis of tumor cells is a complex process which includes a complex cascade of events. Included among these events are the proliferation of tumor cells and blocking of the apoptosis mechanism (Harrington, E. A., et al., EMBO J. 13:3286-3295 (1994)), the spread of the tumor cells into surrounding tissues, the penetration of tumor cells into the blood and lymphatic circulations, and the attachment and multiplication of tumor cells at a new site (Schirmacher, A. E., Adv. Cancer Res. 43:1-42 (1985); Liotta, L. A., et al., Lab. Invest. 49:636-649 (1983)). While the molecular mechanisms of the induction of the metastatic phenotype remain poorly studied, it is likely that activation and/or inactivation of a variety of regulatory and structural genes occur as a tumor becomes metastatic. Several genes whose expression correlates with the metastatic potential of tumors have been described (Yasushi, T., et al., J. Biol. Chem. 269(37):22958-22963 (1994); Ebralidze, A. K., et al., Genetika (USSR) 25(5):932-936 (1989); Wolf, C., et al., Proc. Natl. Acad. Sci. USA 90:1843-1847 (1993); Bernhard, E. J., et al., Proc. Natl. Acad. Sci. USA 91:4293-4297 (1994); Sato, H., et al., Nature 370:61-65 (1994)).
It has been suggested that the population of cells within a given tumor may be heterogeneous with respect to their metastatic potential (Fidler, I. J., and Hart, I. R., Science 217:998-1001 (1982)). This suggestion offers the possibility of obtaining related tumors that differ markedly in metastatic potential from one parent tumor. For example, tumors that are transplantable to mice and that have a varying frequency and organ-specificity of metastasis have been obtained (Senin, V. M., Vestin. Akad. Med. Nauk. SSSR 0(5):85-91 (1984)) as a result of selection for the metastasis character.
Tumor-Inhibiting Polypeptides
A number of polypeptides naturally produced by mammalian cells have been shown to have anti-tumor activity (i.e., inducing growth arrest, apoptosis and/or differentiation of cancer cells). For example, combinations of certain cytokines, such as interleukins and colony-stimulating factors, have been reported to induce terminal differentiation and concomitant growth arrest of particular types of tumor cells and cell lines (reviewed by Pimental, E., in: Handbook of Growth Factors, Vol. 1, Boca Raton, Fla.: CRC Press, pp. 28-34 (1994)). In addition, transforming growth factor .beta. (TGF-.beta.) and the interferons are known to be potent growth inhibitors for tumor cells under certain conditions in vivo and in vitro (Keski-Oja, J., and Moses, H. L., Med. Biol. 66:13-20 (1987); Ohta, M., et al., Nature 329:539-541 (1987)). Other natural mammalian polypeptides having a variety of tumoricidal activities, such as the induction of apoptosis in certain tumor cells, include the tumor necrosis factors (TNFs) (reviewed by Pimental, E., in: Handbook of Growth Factors, Vol. 3, Boca Raton, Fla.: CRC Press, pp. 241-278 (1994)).
Recently, a family of TNF-related cytokines and their receptors with somewhat common structural features has been identified. This family includes the TNF, LT-.alpha., LT-.beta., Fas, CD27, CD40, OX-40, and nerve growth factor (NGF) receptor-ligand systems (Smith, C., et al., Cell 76:959-962 (1994); Armitage, R. J., Curr. Opin. Immunol. 6:407-413 (1994)). With the exception of NGF, all of these TNF-related cytokines are thought to be involved in the regulation of the immune system. TNF and lymphotoxin-alpha (LT-.alpha. or TNF-.beta.) are related cytokines involved in many regulatory activities (Vassalli, P., Ann. Rev. Immunol. 10:411-452 (1992); Paul, N., and Ruddle, N., Ann. Rev. Immunol. 6:407-438 (1988)) but their roles in the immune system, while apparently critical, remain enigmatic (de Kossodo, S., et al, J. Exp. Med. 176:1259-1264 (1992)). TNF is synthesized in response to various insults by a variety of cell types; this is generally regarded as one of the primary events in the inflammatory cascade, including a potent antitumor effect on mice (Blankenstein, T., et al, J. Exp. Med 173:1047-1052 (1991)). Activated macrophages are the major source of membrane-bound TNF, although it is also produced by activated lymphocytes and several other cell types. In contrast, LT-.alpha. is produced specifically by lymphocytes and exists in membrane-associated form only via a trimeric complex with LT-.beta. (Browning, J., et al., Cell 72:847-855 (1993)). LT-.beta. shows a spectrum of activities similar to that of TNF in in vitro systems, but is less potent (Browning, J., and Ribolini, A., J. Immunol. 143:1859-1867 (1989)). Both TNF and LT-.alpha. induce apoptosis in various systems (Cohen, J. J., et al, Ann. Rev. Immunol. 10:267-293 (1992); Golstein, P., et al., Immunol. Rev. 121:29-65 (1991); Sarin, A., et al., J. Immunol. 155:3716-3718 (1995)). Recently, tumor growth inhibition mediated by LT-.alpha. has been reported (Qin, Z., and Blankenstein, T., Cancer Res. 55:4747-4751 (1995)).
TNF and LT-.alpha. are also released by a number of tumor cells of various origins, such as mouse fibrosarcomas, human epithelial cell lines and T-cell leukemia cells and cell lines (Rubin, B. Y., et al., J. Exp. Med. 164:1350-1355 (1986); Spriggs, D. R., et al, J. Clin. Invest. 81:455-460 (1988); Ishibashi, K., et al., Blood 77:2451-2455 (1991)). The genes encoding TNF, LT-.alpha. and LT-.beta. lie closely spaced within the class III region of the major histocompatibility complex (MHC) (Spies, T., et al., Proc. Natl. Acad. Sci. USA 83:8699-8702 (1986); Nedospasov, S. A., et al., Nucl. Acids Res. 14:7713-7725 (1986); Gardner, S. M., et al., J. Immunol. 139:476-483 (1987)). These genes are thought to be evolutionarily related and are though to have formed the locus by tandem gene duplications, although the opposite orientation of LT-.beta. transcription suggests that more complex evolutionary events may have taken place. Recently, another novel TNF family member has been cloned and designated as TNF-related apoptosis-inducing ligand (TRAIL) (Wiley, S. R., et al., Immunity 3:673-682 (1995)).